Josephson tunneling studies of odd-parity superconductivity in Sr$_2$RuO$_4$

In the recent phase-sensitive work that provided the most unambiguous evidence for odd-parity superconductivity in Sr$_2$RuO$_4$, we used Au$_{0.5}$In$_{0.5}$ as the s-wave superconducting counter electrode in the SQUID structure. However, Au$_{0.5}$In$_{0.5}$ has a T$_c$ less than that of Sr$_2$RuO$_4$, making detection of the pairing symmetry near the T$_c$ of Sr$_2$RuO$_4$ inaccessible to the phase sensitive measurements. To go beyond this limit and open up possibilities of several other experiments involving Josephson tunneling into Sr$_2$RuO$_4$, we seek an alternative material system with an s-wave superconductor of T$_c$ $>$ 1.5K that would exhibit Josephson coupling with Sr$_2$RuO$_4$. An Ag/Pb/Ag trilayer has been chosen for this purpose. Ag/Pb/Ag-Sr$_2$RuO$_4$ tunneling devices were prepared that showed a suppressed superconducting gap feature of Sr$_2$RuO$_4$, suggesting the presence of superconductivity at the polished ac face of a Sr$_2$RuO$_4$ crystal. However, no Josephson coupling between Ag/Pb/Ag and Sr$_2$RuO$_4$ was detected. More experiments are currently underway, and new results will be presented.